The invention relates to a method for operating a compression ignition internal combustion engine.
Two of the objectives in the development of new diesel internal combustion engines are to reduce the fuel consumption and to minimize exhaust emissions, in particular the emissions of nitrogen oxides and the formation of particulates. In modern compression ignition internal combustion engines, exhaust gas recirculation is often used as a means for lowering emission levels, with an exhaust gas recirculation rate being set as a function of the load point.
The nitrogen oxide emission levels can be lowered further by using an SCR catalytic converter, in which the addition or metering of a reducing agent, e.g., ammonia, is implemented proportionally to the formation of nitrogen oxides in the internal combustion engine. The required safety devices only permit limited conversion rates in an SCR catalytic converter of this type of between 60% and 70%, since the untreated nitrogen oxide emissions from the internal combustion engine can only be determined from known engine map data. At present, sensors for the direct measurement of the concentrations of nitrogen oxides or ammonia in the exhaust gas are still in the research stage, since sensors of this type are currently still excessively inaccurate or extremely sensitive to external influences.
The fuel economy of an internal combustion engine generally results only indirectly from the engine maps for the respective injection point stored in the engine control unit. There is no feedback or correction based on the current efficiency or fuel consumption of the internal combustion engine.
DE 197 34 494 C1 has disclosed a method for operating an internal combustion engine, in which a recirculation rate of the exhaust gas is calculated on the basis of a two-fold measurement of the oxygen concentration in the exhaust gas and in the charge air. In this method, in addition to the fact that the outlay on measuring equipment is high, only the recirculation rate of the exhaust gas is determined.
DE 100 43 383 C2 has disclosed a method for determining the nitrogen oxygen content in exhaust gases from internal combustion engines, in which the air mass fed to the internal combustion engine is recorded, with the combustion center of gravity being determined from at least one current measured value for the engine operation. The untreated NOx emissions are calculated from the value for the position of the combustion center of gravity and the values for the recorded fuel quantity and air mass.
The combustion center of gravity uses the first law of thermodynamics to describe the state in the combustion chamber in which 50% of the fuel energy introduced has been converted. The position of the center of gravity is the associated crank angle position, i.e. a crank angle position of the piston, at which 50% of the quantity of fuel participating in the combustion has been converted into heat.
The considerable outlay involved in the simultaneous determination of air, fuel and exhaust gas recirculation rates is disadvantageous, since a combustion chamber pressure profile resolved on the basis of the crank angle is required for the calculation of the combustion center of gravity, and this pressure profile is complex to determine by metrology.
Therefore, the invention is based on the object of providing a method for controlling an internal combustion engine which ensures consumption-optimized operation of the internal combustion engine combined, at the same time, with a drop in the NOx emissions.
According to the invention, this object is achieved by a method for operating a compression-ignition internal combustion engine. The method includes metering in a quantity of fuel as a function of an operating point of the engine during a working cycle, and injecting the quantity of fuel which is metered in into the combustion chamber in such a manner that a combustion center of gravity is positioned at a defined crank angle position independently of the operating point of the internal combustion engine.
In a method according to the invention, a quantity of fuel is metered in as a function of the operating point during a working cycle of the internal combustion engine is the metered quantity of fuel being injected into the combustion chamber in such a manner that a position of each combustion center of gravity is at a predetermined crank angle position independently of the operating point of the internal combustion engine.
According to the present invention, the efficiency of the compression ignition internal combustion engine is directly related to the position of the combustion center of gravity. In this case, the engine parameters are set in such a manner that the position of the center of gravity is at a defined crank angle position independently of the operating point currently being run during the respective combustion or during each combustion. This crank angle position can be determined for the respective internal combustion engine before it starts to operate, e.g., on an engine test bench. A maximum efficiency is achieved at this position of the combustion center of gravity which has been predetermined for the respective internal combustion engine. This predetermined position of the center of gravity should then as far as possible be maintained throughout the entire service life of the internal combustion engine.
According to a further configuration of the invention, a current position of the combustion center of gravity is determined as a function of a recorded pressure profile in the combustion chamber, the pressure profile in the combustion chamber preferably being recorded by a sensor. This achieves accurate determination of the position of the center of gravity. For this purpose, it is preferable to determine the current value of the center of gravity position of the combustion with the aid of a calculation model, so that the metrology outlay can be reduced further.
In a further configuration of the invention, the current combustion center of gravity is determined as a function of a crank angle position at which a maximum cylinder pressure is recorded in the combustion chamber. Accordingly, the center of gravity position of the combustion is determined with the aid of an empirical model by a point in time at which the maximum pressure in the cylinder, e.g., an ignition pressure, is reached. According to the present invention, the center of gravity position is dependent on the crank angle position of the maximum pressure occurring in the combustion chamber. This significantly simplifies determination of the combustion center of gravity, since there is no need for processing or detailed resolution of the entire cylinder pressure profile in the combustion chamber on the basis of the crank angle during combustion.
According to a further configuration of the invention, the current combustion center of gravity is determined as a function of a fuel injection duration, the start of fuel injection, a charge mass in the combustion chamber and the speed of the internal combustion engine. In this context, the charge mass can be taken from the engine maps stored in the engine control device, in order to simplify the method. As a result, determination of the combustion center of gravity using an empirical model is further simplified. Therefore, rapid calculation or determination of the position of the combustion center of gravity can be achieved without having to use complex sensors in the combustion chamber.
According to a further configuration of the invention, an exhaust gas recirculation quantity for setting a defined oxygen concentration in the combustion chamber is set as a function of the center of gravity position of the combustion. In this case, the required exhaust gas recirculation rate is calculated from the determined untreated NOx emission from the internal combustion engine, and the exhaust gas recirculation is controlled until a defined oxygen concentration is established in the combustion chamber. It is preferable for a desired value for the oxygen concentration to be stored as a constant value in an engine map for the internal combustion engine stored in the engine control device.
In one configuration of the method according to the invention, the position of the combustion center of gravity is set by varying the start of the compression ignition and/or by varying the fuel injection. This results in targeted and rapid control of the internal combustion engine at the respective load point, so that the internal combustion engine can operate with a maximum efficiency and reduced formation of untreated NOx emissions.
Further features and combinations of features will emerge from the description. Specific exemplary embodiments of the invention are illustrated in simplified form in and explained in more detail with reference to the following drawings.